Track whether whole-arrays are assignable

In GLSL 1.10 this was not allowed, but in GLSL 1.20 and later it is.

This causes the following tests to pass:

    glslparsertest/glsl2/array-09.vert
    glslparsertest/glsl2/array-13.vert
diff --git a/ir.h b/ir.h
index 3caff36..5267d2b 100644
--- a/ir.h
+++ b/ir.h
@@ -146,6 +146,14 @@
 
    unsigned mode:3;
    unsigned interpolation:2;
+
+   /**
+    * Flag that the whole array is assignable
+    *
+    * In GLSL 1.20 and later whole arrays are assignable (and comparable for
+    * equality).  This flag enables this behavior.
+    */
+   unsigned array_lvalue:1;
 };